1. Field of the Invention
The present invention relates to light leakage preventing devices for photographic cameras provided with focal plane shutters and more particularly to a device for preventing a light from leaking onto a film surface from within a view finder optical system in a focal plane shutter of a type in which a part of shutter blades will advance into the view finder optical system at the time of the shutter operation.
2. Description of the Prior Art
Generally, in a photographic camera provided with a focal plane shutter, as a view finder optical system and a shutter device are perfectly isolated from each other, a light can not leak onto a film surface through the shutter device form the view finder optical system. However, for example, as disclosed in U.S. Pat. No. 3,999,196, there have been defects that, in the case of a focal plane shutter of a type in which a part of shutter blades is so arranged as to advance into a view finder optical system of a photographic camera at the time of the shutter operation in order to reduce the height of the camera, a port for the advance of the shutter blades into the view finder optical system will be required and therefore a part of the light passing through the view finder optical system through this port will advance into the shutter device and, as a result, an undesirable exposure will be performed on the film surface.
These defects shall be concretely explained in the following with reference to a conventional example schematically shown in FIGS. 1 to 3. In FIG. 1, reference numeral 1 indicates a view finder optical system provided on the upper part of a camera body B and consisting of a penta-prisim 2, a lens group 3 and a space 4 formed between them. Numeral 5 indicates a shutter device positioned just before a film F below the view finder optical system 1. The shutter device 5 consists of base plates 6 and 7 and a partition plate 8 arranged parallelly with each other with a predetermined spacing between them, a front shutter blade group 9 consisting of a plurality of opaque thin plates foldably and developably supported within a space defined by the base plate 6 and partition plate 8, a rear shutter blade group 10 of a plurality of opaque thin plates foldably and developably supported within a space defined by the partition plate 8 and base plate 7 and a driving means not illustrated provided on the base plate 6 to move the front shutter blade group 9 and rear shutter blade group 10 between the folded position and unfolded position. Exposure apertures 6a, 7a and 8a coordinated on an optical axis are formed respectively in the base plates 6 and 7 and partition plate 8. Numeral 11 indicates a well known reflecting mirror supported rotatably on the camera body B and serving to introduce a light from an object to be photographed into the view finder optical system 1.
FIG. 1 shows a state after an exposure is completed and before the shutter is cocked. In this state, the front shutter blade group 9 and rear shutter blade group 10 are unfolded to cover the exposure apertures 6a, 7a and 8a. The reflecting mirror 11 is in a position to lead a light from an object to be photographed into the view finder optical system. If the shutter is cocked in this state, through the driving means not illustrated, the rear shutter blade group 10 will be moved to a position to be folded as shown in FIG. 2, that is, to open the exposure apertures 7a and 8a and will be locked in this position. Thus, the driving means for the rear shutter blade group 10 will be held in the cocked position. At the same time, the driving means for the front shutter blade group 9 will be also moved to the cocked position and will be held in this position. In this case, the front shutter blade group 9 will remain held in the position in FIG. 1.
If a release button not illustrated of the camera is pushed in this state, first, the reflecting mirror will be sprung up as shown in FIG. 3, the light from the object to be photographed led into the view finder optical system 1 will be intercepted and, instead, will advance toward the exposure apertures 6a, 7a and 8a. Then, the driving means for the front shutter blade group 9 will be released and the front shutter blade group will move upward to open the exposure apertures 6a and 8a. Thus, the exposure to the film F will be started. In this case, the front shutter blade group 9 will be folded as shown in FIG. 3, will advance into the space 4 in the view finder optical system 1 and will stop. Then, when a certain time (exposure time) elapses, the driving means for the rear shutter blade group 10 will be released, the rear shutter blade group 10 will move to cover the exposure apertures 7a and 8a and thus one exposing operation will be completed. In the final step of the above mentioned movement of the rear shutter blade group 10, the front shutter blade group 9 will again return to cover the exposure apertures 6a and 8a and the reflecting mirror 11 will be also returned to the position in FIG. 1 in association with this returning motion. Thus, the light from the object to be photographed will be again introduced into the view finder optical system 1.
In a single lens reflex camera provided with such socalled quick return type focal plane shutter as is described above, an advancing port through which the front shutter blade group 9 advances into the space 4 in the view finder optical system 1 is required in the mechanism. In this kind of conventional camera, there have been defects that, as this advancing port is always open, a part of the light passing through the view finder optical system 1 will enter the shutter device through the above mentioned advancing port and, as a result, the film will be exposed by the light.